The technical field of this invention is low resolution Raman spectroscopy and, in particular, the invention relates to analysis of a constituent or a property of a sample using low resolution Raman spectroscopy. The sample can be an liquid, such as an organic liquid. Alternatively, the sample can be a solid, such as a powdered drug.
It is known in the art that the chemical analysis of a liquid containing organic components either as the main constituent (e.g., hydrocarbon fuels, solvent mixtures, organic process streams) or as a contaminant (e.g., in aqueous solutions) can be based upon optical spectrum analysis of that liquid. The optical spectral analysis used can be near infrared (IR) analysis, despite its inherent low resolution. Near IR chemical analysis systems use inexpensive light sources and detectors. Advantageously, these systems also use optical fibers to deliver excitation radiation to the sample and to collect the resulting signal. In addition, chemometric analysis of broad spectral features improves the resolution of such near IR systems.
In contrast, mid IR analysis provides easily identifiable spectra for many samples of interest. Mid IR provides a "fingerprint" spectral region having sharp detail. The sharp detail of the fingerprint spectral region makes subsequent analysis easier.
Raman spectroscopy provides many of the advantages of near IR. Raman spectroscopy can also provide detailed spectral analysis, typical of mid IR spectroscopy, for organic systems, particularly for liquid organic systems. However, one drawback to Raman spectroscopy has been its expense relative to mid and near infrared systems.
A significant component of that expense is the laser system required to produce quality, high-resolution spectra. Even using a laser diode as the scattering source, the laser remains one of the major expenses in developing cost-effective Raman systems.
U.S. Pat. No. 5,139,334 issued to Clarke, and incorporated herein by reference, teaches a low resolution Raman spectral analysis system for determining properties related to the hydrocarbon content of fluids, in particular, the octane rating of gasoline. The system utilizes a laser Raman spectroscopic measurement of the hydrocarbon bands and relates specific band patterns to the property of interest. Different fuel properties are determined by a method that compares Raman-scattered light intensities over different wavelength ranges.
However, there remains a need for a low cost, portable, low resolution Raman spectroscopic system that does not depend on large and highly complex mechanical devices. Moreover, there exists a need for a low-resolution Raman spectroscopic system that provides a high intensity signal.